In telecommunications, in particular radio frequency signal processing, the objective of generating a defined output level from a variable input level may frequently be encountered. Amplifiers having variable gain (which are also referred to as Variable Gain Amplifiers (VGA)) are normally used for this purpose.
VGAs of this type may have either a dB-linear or a voltage-linear characteristic curve, the latter being distinguished by the fact that they have a gain factor which is linear with respect to a reference variable, namely a control voltage.
The document by Alan Grebene: “Bipolar And MOS Analog Integrated Circuit Design”, ISBN 0-471-08529-4 shows an example of a conventional VGA architecture in figure 8.36 on page 446. The so-called AGC (Automatic Gain Control) amplifier of broadband design shown there is based on the structure of a so-called Gilbert multiplier cell. In this case, provision is made of two differential amplifier cells which are connected in parallel on the control side, the desired gain being applied as reference variable to the differential input formed in this way. The base points of the two differential amplifier cells are connected to outputs of a further differential amplifier, which likewise has control inputs having a differential input for a signal to be amplified.
An amplified output signal—likewise present in differential form—can be discharged at the outputs of the two differential amplifier cells, said outputs being connected up to one another in a suitable manner. In other words, a radio frequency input signal which has been applied, at the base point, to the control inputs of the transistors of the differential amplifier is reversed in the quartet of four transistors of the differential amplifier cells. In this case, a variable signal current component from one transistor of each of the two differential amplifier cells is conducted to an electrical load, while the remaining partial currents of the respective other differential amplifier cell transistors are conducted directly to the supply potential and are thus discarded. A structure of this type having a reference variable is usually driven using a so-called VGA buffer, to be precise in such a manner that the transistors of the differential amplifier cells are simulated there and are connected up as diodes in order to generate voltages via variable currents on their diode characteristic curves, said voltages in turn serving as a reference variable, namely as a control voltage for the gain of the amplifier having variable gain. The currents to be set are in turn controlled using a guide voltage UGC (gain control voltage). A buffer structure of this type itself has a hyperbolic transfer function caused by the current-voltage conversion on the diode characteristic curves of the simulated transistors. If a linear behavior of the gain factor is desired, this is possible approximately only in a narrow central operating range.
In order to nevertheless achieve a wide dynamic range, a VGA could be embodied in a plurality of stages. However, this would be associated with the disadvantage of a large area occupation when embodied using integrated circuit technology in conjunction with a likewise disadvantageous high current consumption. However, the disadvantage of the single-stage embodiment explained is that, in the case of a high dynamic response requirement, the four transistors of the two differential amplifier cells of the Gilbert multiplier have to be reversed within a very wide range. This means that the diodes modeled on these transistors in the VGA buffer also have to be reversed within a wide range. If high attenuation of the radio frequency input signal is desired in this case, the output transistors and thus also one of the diodes assigned to the latter in the buffer must become virtually de-energized. However, in this virtually de-energized operating state, manufacturing fluctuations during the production of integrated circuits and also temperature fluctuations lead to particularly great changes in the gain factor selected. When the gain is considered logarithmically, this disadvantageous behavior has such an effect that it is practically no longer possible to correctly set a low gain. If the gain factor tends toward zero, the gain tends practically toward minus infinity. Yet another disadvantage of the known structure described is that the diodes in the VGA buffer have to have the same current density as the quartet of transistors of the differential amplifier cells. If the VGA is to enable high output powers, the largest current component of the VGA buffer is in the two transistor diodes mentioned.
It may be desirable to design a control loop having an amplifier such that the gain factor of the amplifier is proportional to a reference variable. In the case of such a control loop, the minimum and the maximum gain is determined by the amplifier with a variable gain factor. In this case, the problem can arise that such a linear gain factor extends as far as the minimum gain factor that can be set, and, if the gain is considered with a logarithmic gain factor, may lead to a very high amplifier slope. If the gain factor tends towards zero, then the gain tends towards minus infinity. Slopes of hundreds of dB per volt may occur in this case. The undesirable consequence would be a severe feedthrough of disturbances such as noise at the control input of the amplifier onto the gain factor set. This would be associated with the disadvantage that when setting very small gain factors, that is to say high attenuations, large fluctuations in the gain would occur in the event of disturbances at the reference variable input of the controller.
A similar problem area could result at the maximum of the variable gain of the gain factor characteristic curve. If an amplifier circuit having a voltage-linear amplifier characteristic curve is required, then a kink in the characteristic curve results in the region of the maximum of the gain, said kink being undesirable. If an amplifier having such a characteristic curve is used for example in a mobile radio transmitter for the purpose of setting the transmission power, so-called switching transients inevitably arise on account of the property of the amplifier characteristic curve described. Said switching transients are undesirable spectral energy components which are generated during variation of the gain along the characteristic curve.